I meant to use a captcha beofore submitting a block that will be a puzzle or something that can be solved by a human brain or a botnet and using captcha on top of it just to be secure and sure that the block was mined by a person.
I like the idea. A block could be solved by working out a maze puzzle, and as difficulty increases the maze becomes more complex.
System should be decentralized: only shared state should be in the blockchain. How would one generate puzzles/captchas without central server?
The hash of the previous block could possibly serve as a parameter for maze generation?